1. Field of the Invention
The present invention relates to a dielectric filter comprising a plurality of juxtaposed resonators, and a duplexer using such resonators.
2. Prior Art
FIG. 1 of the accompanying drawings illustrates a known typical dielectric filter comprising a dielectric ceramic block A, three or more than three resonators B provided in the block A respectively by coating the peripheral walls of three or more than three through bores with a conducting material, an external conductor C covering the outer surfaces of the dielectric ceramic block A but not open-circuit end surface having the openings of the through bores, a pair of coupling through bores D arranged between adjacent ones of the resonators B in parallel with the latter and having no coating on the peripheral walls thereof and a pair of input/output pads E arranged on a lateral surface of the dielectric ceramic block A at respective positions abutting the open-circuit end surface of the block A and facing the respective outermost resonators in such a way that they are electrically insulated from the external conductor C. With such arrangement the magnetic field coupling intensity of the adjacent resonators may be intensified because dielectrics are cleared off between adjacent resonators B by a coupling through bore D. A variety of such dielectric filters are currently known.
With such a known arrangement, however, a fly back x (see FIG. 4B) is produced at an attenuation pole in a higher frequency zone of a resonance frequency band under the influence of the coupling through holes D to hold the output level above the threshold value of -50 dB as shown in FIG. 4B and reduce the attenuation w to such an extent that the filter does not operate satisfactorily in terms of attenuation and is apt to generate noise.
It is therefore an object of the present invention to provide a dielectric filter that is free from any degradation in the attenuation effect of the filter due to a fly back x.
Another object of the present invention is to provide a duplexer using such dielectric filter.